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ANALYTICAL CHEMISTRY, VOL. 50, NO. 4 , APRIL 1978
CORRESPONDENCE Construction of pH Gradients in Flow-Injection Analysis and Their Potential Use for Multielement Analysis in a Single Sample Bolus #Sir: We are carrying out a long term investigation into the possibility of performing multielement trace analysis by nonsegmented continuous flow analysis, which is more concisely known as flow-injection analysis (FIA) ( I ) . The rationale and some preliminary results are reported here. Basic Idea. The distinctive feature of FIA, compared to conventional automatic analysis, is the direct injection of a sample into the carrier solution. It follows that there is an interfacial region between the sample plug and the carrier and that, during the course of mixing, concentration gradients are established across the interface if the initial concentrations in the carrier and sample are different. Thus, if we have a system in which the sample solution is a mixture of metal ions and t h e carrier is a solution of a reagent which reacts with the metal ions, the sample and carrier being of different pH, then we would expect a well defined sequence of color-forming reactions t o take place across the interfacial region as the reagent and metal ions reacted and the p H changed. Under static conditions, pH-absorbance curves (Figure 1) can be readily established and are known to be characteristic for any given metal ion-reagent combination. They are additive, and of such a shape that if the curve for a mixture of ions were mathematically differentiated, it would give rise to a well defined series of peaks, whose relative areas reflected the composition of the original mixture. Under the dynamic conditions of FIA, the chemistry becomes complicated, b u t Figure 1 suggests the crucial experiment t o test whether concentration changes across the interface can be used for analytical purposes. If the carrier is a solution of 4-(2-pyridylazo)resorcinol,PAR, a t p H 9 and t h e sample bolus is a solution of lead(I1) and vanadium(V) at p H 2, the p H across the bolus-carrier interface will vary between 9 and 2. At p H 9, only the lead will react with the PAR whereas a t p H 2 only vanadium forms a color. Hence, the peak obtained by measuring the absorbance downstream from the injection point will not have the Gaussian shape which is obtained when the sample solution contains only a single metal ion, and it might contain enough information for the two ions to be determined.
EXPERIMENTAL Reagent. 4-(2-P~.’ridylazo)resorcinol.Reagent streams are prepared freshly from reagent grade PAR diluted to M. Ammonia-Ammonium Chloride Buffer. The reagent stream is buffered at pH 9.9 by dissolving 12.2 g of ammonium chloride and 50 mL of 35% ammonia solution in each liter of reagent solution. Samples. Samples were prepared for stock solutions of AnalaR ammonium metavanadate in dilute hydrochloric acid (0.025 M) and AnalaR lead nitrate. Each sample was prepared as a solution of 0.025 M hydrochloric acid. Apparatus. Constant Head. The stream was motivated by a constant head of reagent at a flow rate of 2.5 mL/min. Sample Injection. Samples were injected from a disposable plastic syringe through a septum valve into the reagent stream. Mixing Coils. Since a large sample volume is used, a coil of 100 cm is required before the injection block to accommodate any backflow of sample on injection. The reaction coil was 215 cm in length and both coils were 0.086 cm i.d. 0003-2700/78/0350-0654$01.0010
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Table I. Effect of Vanadium Concentration on Lead Peaks Concn Pb(II), 10-5 M
Concn V(V), l o - 5M
5.4 5.4
5.4 5.4
6.7 13.3 20.0 26.7
Peak height of
lead peak, cin
17 16 16 16.5
.Spwtrophotomrter. The peaks were detected using a 1-mm Perspex cell in a Unicam SP600 spectrophotometer set a t 530 rim, the flow direction being normal to the light path. Procedure. In view of the known solution chemistry of vanadium and lead and the need to establish an optimal pH gradient across the bolus-carrier interfaces, it is important to maintain close control of the hydrochloric acid and ammonia-ammoniwn chloride buffer constituents and to use recently prepared solutions of vanadium(V) and buffered reagent solution. Each sample solution was prepared as a solution of 0.025 hl hydrochloric acid. The vanadium solutions were used within 24 h of their preparation. Sample volumes of 1.25 mI, were used in all experiments. Sample batches were prepared to establish the effect on the peak height for an analyte when its concentration was varied in the presence of (a) a constant concentration of the second analyte, (b) a varying concentration of the second analyte, and (c) when its concentration was held constant while the second analyte concentration was varied.
RESULTS AND DISCUSSION Typical peak profiles are shown in Figure 2 . The outer peaks arise from the reaction of PAR with lead while the inner peak is due to vanadium. A straight line calibration is obtained for lead in the presence of a constant concentration of vanadium (Figure sa). No effect is detected in the lead peak obtained with a constant concentration of lead when varying the concentration of vanadium, the peak heights all being within the expected precision limits of a single sample, viz. standard deviation (1-270) (Table I). A straight line calibration is obtained for the lead peaks of varying concentration in the presence of varying vanadium concentrations (Figure 3b). Clearly lead can be determined selectively in the presence of vanadium. The resolution of the lead and vanadium peaks as shown in Figure 2 is greatest when the vanadium concentration is reasonably high. This puts the range of concentration of vanadium over which determinations can be made out of the linear range obtainable with the PAR concentration. Hence the determination of concentration is much less precise. The calibration curves which can be obtained (Figure 4) are in the region of the break point similar to that which is obtained in conventional mole ratio plots. (See Theoretical discussion.) In this case the lead concentration has been kept constant. When the lead concentration is varied, t h e peaks for lead and vanadium overlap, significantly at high lead concentration. I t is no longer possible simply to measure the peak height of the vanadium peak from the baseline. Since the calibration curve for vanadium is very shallow locating the “base“ of the C 1978 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 50, NO. 4,APRIL 1978
i
E
P
2
655
Lot
Y 0
a 0
I
2
3
5
4
6 PH
8
7
9
1c
I1
Figure 1. pH-absorbance curves for lead(I1)and vanadium(\/) with 4-(2-pyridylazo)resorcinol. Pb2+ = V5+ = 2 X M; PAR = 2 X M; wavelength, 530 nm
a.
b.
C.
Figure 2. Peaks obtained from one sample containing a mixture of Pb(I1) and V(V) injected into a solution of PAR. (a) General shape of curve. (b) Increasing concentration of V(V) while that of Pb(I1) is held constant. (c) Increasing concentration of Pb(I1) while that of V(V) is held constant
J
’’
/
,
-
015
15
’
2
concentratio? PbII lO--K
ci
1
2 3 L 5.: csncer.:ration PbII ’ 0
P
5
Figure 3. (a) Peak height of lead peak vs. Pb(I1) concentration with V(V) concentration constant at 5.0 X M. (b) Peak height of lead peaks vs. Pb(I1) concentration with V(V) concentration varying randomly. For points from left to right, the V(V) concentration is 7.1, 11.1, 2.5, 2.9, 8.9, 6.7 X
M
vanadium peak must be very precise. It is for this reason that the calibration curves obtained for vanadium in the presence of varying lead have not been useful. We believe, however, that these results demonstrate the feasibility of using unsegmented continuous flow systems to obtain a range of conditions (pH gradient, masking gradient, etc.) over a single sample bolus so that resolution of absorbing species in multielement determination may be achieved. It would seem certain from the results presented and difficulties high-lighted that some modicum of computing power will be required for peak resolution and more precise determination of peak characteristics. Theoretical Consideration. The basic assumption made is that when there is a difference in the initial concentrations of a species in the carrier and sample solution, a concentration gradient is established in the interfacial region. The reasonableness of this assumption is suggested by Taylor’s theory on the dispersion of a plug of sample injected into a flowing solvent (2). In essence, the theory is that a t low flow rates and within specified dimensions of tubing, the dispersion of the sample into the carrier is mainly by diffusion and that the concentration profile of the sample bolus downstream is Gaussian. Thus, the concentration gradient, although nonlinear is mathematically predictable and, more important, is reproducible. In other studies in this laboratory, we have
0
3
2
:CM Figure 4. Peak height of vanadium peak vs. V(V) concentration with Pb(I1) concentration constant at 5.4 X M coicentra:ion
V!‘J J
found that Taylor’s theory fits the experimental results very well. It is a good first approximation even when, as in this instance, the flow rate is slightly greater than allowed for in the theory. This theory based on laminar flow seemed more suitable than one which assumed turbulent flow, since the flow rate which corresponds to a Reynolds number of 2000, usually set as the onset of turbulence, is 81.3 mL min-’. Ruzicka and Hansen, a t first recommended turbulent flow, but soon abandoned that viewpoint and have recently proved that it is inessential to FIA ( 3 ) . It is not even certain that a change to turbulent flow would upset the argument greatly since Taylor has shown that diffusion is important under these conditions and that peak shapes are predictable ( 4 ) . However the experimental arguments overwhelmingly favor slow flow rates, and the discussion about turbulence is a little academic. We have checked the possibility that some of the peak spreading arises from chromatographic processes, by performing the experiment a t different flow rates. The results clearly indicate that the effective partition coefficient approximates to zero and that diffusion is the major cause of peak spreading. In general, the identification and determination of elements in a mixture may be made easier by a “substoichiometric sharpening effect”. It seems probable that a substoichiometric amount of reagent presents itself to the metal ion at any given point across the interface. Thus the reaction which is favored a t that point, is the one with the greatest conditional equilibrium constant. Hence, in general, one would expect metal ions to be complexed sequentially, not simultaneously. As the p H varies, so do the conditional constants, and, in those instances where the relative magnitude of the constants is reversed (as here a t approximately p H 7 ) , one would expect the effect observed. This argument may seem a little farfetched since in the system there is clearly an excess of reagent overall. However, three kinetic factors govern the amount of reagent which can react with the sample by the time the plug reaches the point of measurement: (1) the rate of flow of carrier, (ii) the rate of mixing of sample and carrier, (iii, the rate of chemical reaction between reagent and metal ion. We have performed experiments equivalent to those used to determine the mole ratio in spectrophotometry, Le., we have injected a given volume of increasingly concentrated solution of lead(11) into a carrier solution of PAR at fixed concentration. Plots of peak height vs. lead concentration are identical t o those obtained in conventional mole ratio experiments (Figure 5 ) . Thus, it is possible to think of an “available amount” of reagent and also of substoichiometry since that amount is not available all a t once. The successful extension of the method to more complex systems depends on the ability to choose suitable combinations of reagents, pH, masking agents, and metal ions, on the ability
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ANALYTICAL CHEMISTRY, VOL. 50, NO. 4,APRIL 1978 30
ACKNOWLEDGMENT We are grateful to J. Ruzicka and E. H. Hanseri for the gift of a sample injection block and t o J. H. Purnell for discussions on the theory of dispersion.
LITERATURE CITED
1
810 a
(I) J. Ruzicka and E. H. Hansen, Anal. Cbim. Acta, 78, 145 (1975). (2) G.Taylor, Roc. R . SOC.(London) Ser. A , , 219, 186 (1953). (3) E. H. bnsen, J. Ruzicka, and B. Rietz, Anal. Chim. Acta, 89. 241 (1977). g O c l (4) G. Taylor, f r o c . R . SOC.(London) Ser. A , , 227, 446 (1953). 02 01, 06 08 10 ' 2 1L 16 18 20 Concentrotlon Pb II
D. Betteridgel Bernard Fields
lO-'M
Figure 5. Peak height vs. Pb(I1) concentration showing the region of limiting reagent concentration. The PAR concentration was 1 0-3 M. (The chelate stoichiometry is Pb(PAR),)
to process the results rapidly, and on the development of a satisfactory theory of FIA. However, we believe these results show that it is possible to create suitable conditions for multielement determinations along a single sample plug.
Chemistry Department University College of Swansea Swansea SA2 8PP, U.K. RECEIVED for review August 9, 1977. Accepted December 27, 1977. The Science Research Council provided a Studentship for B.F.
Gas Chromatographic Determination and Purification of Carbon Diselenide Sir: We have recently had occasion to look into the purity of commercially prepared carbon diselenide in conjunction with our study of its high temperature pyrolysis ( 1 ) . Since carbon diselenide is highly noxious and poisonous, it must be handled with considerable caution. It is not surprising that there are few commercial suppliers, that neither the nature nor amounts of impurities in commercial samples is well known, and that no suitable means of purification have been described. The major commerical supplier (Strem Chemicals, Inc., Danvers, Mass.) reports ( 2 ) preparation of carbon diselenide by the method of Ives, Pittman, and Wardlaw ( 3 ) , ie., by passing methylene chloride vapor over molten selenium metal: ZSe(mo1ten)
+ CH,Cl,(g)
+
CSe,(g)
+
TOLUENE
1
BHCI(g) ,
The carbon diselenide is distilled at 13 Torr (boiling point 45 "C) with a packed column, quick-frozen in a dry ice bath, and stored at -20 "C in the dark to prevent decomposition. Even when kept in this manner, CSe, turns from a clear, lemonyellow color to orange within a couple of weeks and finally to black as solid CSe, polymers form ( 4 ) . When CSez is vaporized at ambient temperature and recondensed, it is again lemon-yellow, the polymeric forms remaining behind. Wagner suggests that methylene chloride is the main contaminant a t the 1 to 5% level (2), possibly due to the formation of an azeotrope as is found in the CS2-methylene chloride system ( 5 ) . Apparently a serious search for other impurities had not been conducted. Marquart, Belford, and Fraenkel ( I ) worked out a convenient procedure to determine the concentration of CSez in a vapor phase by ultraviolet absorption spectrometry in the region of an intense electronic band (2150-2500 A). (NOTE: The instability of CSez in UV light necessitates rapid handling.) Elemental Analysis. Our elemental analysis (performed by the University of Illinois Microanalytical Laboratory under the direction of Josef Nemeth) on a sample of CSe2(1)showed 7.63% C, 0.21% H, and 2.61% C1 indicating that the commerical sample is about 95% pure and that chlorinated species are prevalent impurities. 0003-2700/78/0350-0656$01 0010
60
80
LOO
120 140 160 ELUTION TEMPERATURE
180
1
1
200
1
220
("C)
Figure 1. Gas chromatogram with flame ionization detection of commercially prepared CSe,. Chromatographic conditions: sample injection, neat liquid CSe,, 0.1 NL; column, 6-foot Pyrex ('/*-in. by 2-mm i.d.); packing, 0.4% Carbowax 1500 on 60/80 mesh Carbopack A (Supelco, Inc.); inlet temperature, 150 'C; column temperature, 5 0 to 175 "C at 10 'C/min; carried gas (helium), 30 mL/min; detector temperature, 150 'C
Gas Chromatographic Analysis. The gas chromatographic analyses of CSe2 required only w r y little sample, which was burned in the flame ionization detector. Only small amounts of selenium were released into the atmosphere, no longer in the form of the volatile and noxious CSe2. Accordingly, no special handling techniques were needed. By gas chromatographic-mass spectrometry, we have isolated and identified four chlorinated hydrocarbons: and methylene chloride, chloroform, 1,1,2,2-tetrachloroethane, tetrachloroethylene (1). The last three are probably produced by side reactions during preparation. Tetrahydrofuran (THF), toluene, OCSe, and SCSe are also present. A typical mass spectrum is shown in Ref. 1. Once peaks had been identified, flame ionization detection gave a simpler and generally adequate means of monitoring CSe2 purity. Figure 1 shows typical results with a Varian C 1978 American Chemical Society
